JP-5-26278-U and JP-7-1350-A disclose rechargeable electric tools having a secondary battery as a driving power source, and a circuit for stopping a power supply to a motor when a battery voltage is lowered to a certain reference voltage that is equal to or higher than a discharge end voltage, thereby preventing the tool from being stopped during a working cycle due to a reduction of the battery voltage in use, and preventing a battery lifetime from being shortened due to an over-discharge to a level where the voltage of the secondary battery is reduced to be equal to or lower than the discharge end voltage. Some rechargeable electric tools also have a warning device that indicates a voltage reduction by means of an LED or a buzzer together with the circuit which controls a stoppage of the power supply.
As a secondary battery to be used in a rechargeable electric tool, a NiCd battery and a NiMH battery are most widely used. However, when a secondary battery of this kind is repeatedly charged in a state of a shallow depth of discharge which does not reach full discharge, it is known that there is a memory effect in which a power supply is stopped at a shallow depth of discharge, whereby an apparent battery capacity is reduced.
In a rechargeable electric tool having the control circuit for stopping the power supply to the motor when the battery voltage is lowered, the power supply to the motor is stopped when a terminal-to-terminal voltage of the secondary battery is lowered to the certain reference voltage, thereby prompting to charge or replace the secondary battery. However, In order to prevent the over-discharge from occurring, the reference voltage is set to be higher than the discharge end voltage of the secondary battery. Therefore, the battery is charged in a state of a relatively shallow depth of discharge. As a result, there is a problem that the battery capacity is gradually reduced due to the memory effect when the battery is repeatedly charged.
Moreover, in a cold and low temperature environment, the battery voltage is lowered. Thus, there is a case where the power stopping circuit activates even when the secondary battery is fully charged, so that the electric tool becomes unusable.